One of the major difficulties in computer generation of images is lighting design. Initially, objects in a scene are created; then, the lighting of the objects must be determined. Lighting design requires the selection of lights from a light space. The light space includes all possible locations and types of lights, as well as directions for directional lights. Determining light locations, types and directions to use for a specific lighting effect can be a tedious process.
Under one approach, as each light is added to the scene, an image is rendered again. The user must be able to specify all of the lighting parameters for each of the lights. The user must also evaluate the lighting and specify any changes to the lights in order to change the lighting effects. Thus, the user must be experienced in lighting design and must be able to determine desired placements for lights. This process can be extremely time consuming. If sophisticated rendering programs, such as ray tracing or radiosity, are used, production of an image based upon the lights takes considerable time. The image rendering process must be repeated each time that the lights are changed.
Another approach attempts to determine lighting parameters automatically to achieve a specified lighting effect. The user must be able to articulate desired illumination characteristics of an image. Again, this requires a sophisticated user experienced in lighting design. It also requires a user who can formulate lighting objectives in an understandable format. In addition to requiring a sophisticated user, existing computer systems and processes which determine lights from the desired illumination limit the lighting possibilities for an image.
For example, "Radioptimization--goal-based rendering", Proceedings of SIGGRAPH 93, pp. 147-54, by Messrs. Kawai, Painter, and Cohen, describes a system for determining lights from subjective impressions of illumination entered by a user. The system uses optimization techniques to determine optimum lighting parameters to meet the entered illumination impressions. However, the user must enter a limited set of possible light positions, which severely limits the region of the lighting space which is considered. Similarly, Messrs. Schoeneman, Dorsey, Smits, Arvo and Greenberg disclose a system in "Painting with Light", Proceedings of SIGGRAPH 93, pp.143-46, which uses optimization techniques to determine lights to achieve certain pixel intensity levels entered by the user. This system requires the user to be able to input the pixel intensity levels for the entire image. It also requires a limited set of light positions to be entered by the user in order to determine the optimum lights.
Another system, disclosed in Poulin and Fournier, "Lights from Highlights and Shadows", Proceedings of the 1992 Symposium on Interactive Graphics, pp. 31-38, allows the user to specify the desired locations of highlights and shadows. This system uses geometric techniques to determine optimum light positions and types in order to achieve the desired highlights and shadows. As with the previously discussed systems, this system requires a sophisticated user who has a set lighting pattern in mind. Adjustments to the specified lighting pattern have to be reprocessed in order to determine a new set of light positions.
Therefore, a need exists for a system for creating image lighting which is easily manipulable by the user to obtain a desired result. A need exists for a system which allows changes in lights to be easily computed for review by the user. A need exists for a system which allows a large set of light positions and types to be considered and combined. Finally, a need exists for a system which is usable by persons having limited experience in lighting design.